1. Field of the Invention
The present invention relates to a control apparatus for a multi-cylinder internal combustion engine. Specifically, the present invention relates to a control apparatus for a multi-cylinder internal combustion engine, the control apparatus performing engine control operations such as a fuel-supply control and an ignition-timing control.
2. Description of the Related Art
Gasoline fuels for internal combustion engines have different volatility characteristics. For this reason, when the internal combustion engine is cold-started with less-volatile heavy gasoline, more fuel is deposited on the walls of the intake ports and on the intake valves. Such deposition results in a leaner air-fuel mixture in the combustion chambers (in the cylinders), and the lean air-fuel mixture makes the combustion state poorer. The engine in the poor combustion state may sometimes discharge more unburned gas and may even cause an unintended drop of the engines speed.
In addition, characteristic variations of components such as the fuel injection valves, the air-flow meters to measure the air volume, the intake-pipe pressure sensors, and the pressure regulators, also cause a lean air-fuel mixture in the combustion chambers, thus resulting in similar problems to those described above.
At the moment when the internal combustion engine is cold-started, the sensors to detect the air-fuel ratio have not been activated yet and, accordingly, the air-fuel ratio cannot be detected for a while. The impossibility of detecting the air-fuel ratio, if combined with the use of the less-volatile heavy gasoline, may result in the production of a lean air-fuel mixture and thereby result in unstable combustion. A common practice to avoid such unfavorable unstable combustion is to increase the amount of fuel injection in conjunction with the use of the heavy gasoline. Such a practice, however, renders the air-fuel mixture richer than the theoretical air-fuel ratio in a case where the engine is started by supplying more volatile light or standard gasoline. The rich air-fuel mixture in this case results in an increase in the amount of discharge of unburned gas (HC).
Various fuel control apparatuses have been developed to address the problem of the fluctuation in the air-fuel ratio attributable to the above-described differences in fuel properties or in component characteristics. One of such fuel control apparatuses prevents the air-fuel ratio from becoming either leaner or richer excessively by controlling the amount of fuel injection as follows. Specifically, the apparatus detects the revolution speed (the angular velocity of the crankshaft by a predetermined crank angle) at every single explosion, and controls, once the engine is started, the amount of the fuel injection for the next explosion in accordance with the change in the revolution speed calculated on the basis of the revolution speed of the current explosion and that of the previous explosion (see, Japanese Patent No. 3861937).
A lean air-fuel mixture supplied to the combustion chambers most probably results in an unstable combustion state and thus increases the amount of change for each explosion in the internal pressure of the cylinder. Accordingly, the fuel control apparatus such as one disclosed in Japanese Patent No. 3861937 controls the amount of fuel injection on the basis of the estimation of the air-fuel ratio of the air-fuel mixture while the air-fuel ratio is estimated by detecting the differences in the revolution speed between cylinders of consecutive explosions (inter-cylinder differences in revolution).
The control apparatus disclosed in Japanese Patent No. 3861937 compares, with a predetermined threshold value, either values of the inter-cylinder differences in revolution or their respective averaging values of inter-cylinder differences in revolution. When a value of the inter-cylinder difference in revolution is larger than the predetermined threshold value, it is determined that the air-fuel ratio is lean. Thus, the control apparatus executes a correction to increase the amount of fuel by a predetermined proportion, and thereby prevents the air-fuel ratio from becoming excessively lean. In contrast, when a value of the inter-cylinder difference in revolution is smaller than the predetermined threshold value, it is determined that the air-fuel ratio is rich. Thus, the control apparatus executes a correction to decrease the amount of fuel by a predetermined proportion, and thereby suppresses an increase in the amount of HC discharge due to the excessively-rich air-fuel mixture.